Distributed sensors constitute an attractive and very promising technique for sensing with long wavelengths of physical parameters such as deformation and temperature. In recent years, the application of sensors based on fibre optic technology and linear effects such as Rayleigh scattering, and non-linear effects such as Raman scattering and Brillouin scattering to instrumentation for all types of civil infrastructures (bridges, tunnels, buildings, dams . . . ), transport (airplanes, railway lines . . . ) and industry (gas conduits, water conduits, oil rigs . . . ) has become increasingly widespread.
Moreover, fibre optic distributed metrology techniques are of great help for evaluating and controlling existing fibre optic cabling, basically for applications in the field of communications. In recent years, demand for installation of fibre optic cable has experienced such dramatic growth that it requires leveraging existing cabling and new installations to a maximum. For this reason, having measurement equipment that will allow distributed metrological examination of the specific characteristics of optical fibre has become increasingly important. The basic parameters to be determined include polarisation mode dispersion (PMD) and chromatic dispersion and parameters derived therefrom, such as null dispersion wavelength (λ0).
Rayleigh scattering occurs in any material due to the interaction of photons with the atoms that compose the material. As a result of this interaction, in the specific case of optical fibre, part of the optical signal returns therethrough towards the emitting source.
The Raman Effect is the absorption and subsequent emission of a photon on interacting with electrons in a material medium wherein energy exchange occurs with said medium, causing the electron pass to a virtual state and generating a new photon having greater or less energy than the incident photon. The loss or gain or energy is explained by the generation of a particle called optical phonon.
The Brillouin Effect is similar to that described as the Raman Effect, except that energy exchange is explained by the generation of an acoustic phonon. Both effects that produce new photons at different frequencies to those of the incident photon (or pump photon) are leveraged in the present invention and are used as distributed amplifiers, as low-signal photons draw energy from the photons generated by these processes throughout their propagation through the optical fibre.
Non-linear Raman and Brillouin scattering phenomena which occur in optical fibre are directly dependent on the temperature variations (Raman and Brillouin) and deformations (Brillouin) experienced by said optical fibre, becoming direct sensing techniques for these magnitudes.
Polarisation Mode Dispersion (PMD) is the broadening that the light pulse guided through the fibre undergoes due to the difference in propagation speed between the two basic polarisation states transmitted (slow mode and fast mode). In the case of uniform single-mode optical fibre, symmetrical and installed in such a manner that the curvatures do not induce birefringence, it should have null PMD. In fact, all random fluctuations of curvatures, fibre asymmetries, etc., cause random fluctuations of birefringence throughout the fibre and therefore differences in light pulse time-of-flight, depending on whether it is oriented towards one polarisation or another. Random fluctuations in fibre birefringence occur in all installed fibres and depend on the physical characteristics of the fibres (refraction index, dopant concentration, non-circularity and core ellipticity . . . ), on the manner in which the fibre has been installed (curvatures, micro-curvatures, pressures and tensions . . . ), on the polarising elements included in the transmission line (filters, isolators, Bragg networks . . . ) and on atmospheric constraints (particularly temperature).
Chromatic dispersion (CD) is a phenomenon that appears as a consequence of the linear propagation of light through the fibre and directly related to the dependence of the propagation constant on frequency ( )(D=d1/d, where λ is the light wavelength propagated through the fibre and β1 is the derivative of β in relation to frequency ω). The effect of chromatic dispersion on the signals propagated through the optical fibre is an undesirable broadening of the light pulses that contain the information. This requires compensation of said broadening in order to maintain disturbance-free communication, due to which it is crucial to know the dispersion curve of the fibres with metrological accuracy. Typical chromatic dispersion curves of commercial optical fibres have a wavelength for which parameter D is null, a very interesting fact when deciding what light wavelengths will be used in communication.
Based on these physical phenomena, different varieties of sensors have been developed over the years, such as those based on OTDR (Optical Time Domain Reflectometer) linear scattering and OFDR (Optical Frequency Domain Reflectometer) and those based on non-linear scattering such as ROTDR (Raman Optical Time Domain Reflectometer), ROFDR (Raman Optical Frequency Domain Reflectometer), BOTDR (Brillouin Optical Time Domain Reflectometer), BOTDA (Brillouin Optical Time Domain Analyzer) and BOFDA (Brillouin Optical Frequency Domain Analyzer).
Likewise, in the case of fibre optic metrology, dispersion distribution measurement techniques have been developed (DC, distribution of associated parameters and PMD) based on POTDR (Polarisation Optical Domain Reflectometer), COTDR (Coherent Optical Time Reflectometer) and TCOTDR (Tunable Coherent Optical Time Reflectometer, also known as OCT-Optical Coherent Reflectometry) and similar frequency domain methods (POFDR, COFDR and TCOFDR).
All of these techniques can be implemented in classical detection and detection based on photon counters. They all have an insurmountable limitation inherent to optical fibre, which is the attenuation suffered by light on propagating therethrough. The measurement length ranges between tens of metres and thirty metres.
The fibre distribution sensing systems currently available in the market are:                FOS-TA: Fibre Optic Sensory Technology and Applications. Distributed Temperature and Strain Sensory (DTS & DTSS) System. Singapore. Maximum measurement range 30 km.        Omnisens: DiTeSt: Distributed Temperature & Strain monitoring instruments. Switzerland. Up to 30 km, SMARTECH markets a similar instrument (Switzerland).        Neubrex Ldt. Japan. NEUBRESCOPE: Pre-Pump BOTDA Technique. Up to 25 km.        AGILENT Distributed Temperature System N4385A/N4386A. Based on Raman scattering in multi-mode fibre. Up to 12 km.        YOKOGAWA AQ8603 Optical Fibre Strain Analyser, based on spontaneous Brillouin scattering.        
For fibre optic distributed metrology:    EXFO Distributed PMD Analysis FTB-5600 based on TCOTR.    EXFO Single-Ended Dispersion Analyser FTB-5700.    GAP Distributed dispersion analyser based on photon counting techniques.    LUCIOL INSTRUMENTS OTDR LOR-200/220 and v-OTDR.    ANRITSU MW9076OTDR.
In all of these systems, in both sensors and metrology, measurement uncertainty increases with distance from the sensing point or measurement, due to the increase, among other things, of the measurement signal-to-noise ratio, with the ensuing error in the measured magnitude.